Sunday, 7 October 2012

Pleistocene (Pt 4): Time of the Woolly Mammoths

The second-to-last Ice Age ended around 0.13 million years ago, a full 95% of the way through the Pleistocene. As I've described in Part 3, it was just one of a series of Ice Ages stretching back nearly two million years, and separated by relatively warm 'interglacials'. For much of this time, European wildlife had had a distinctly 'African' flavour, with lions, hyenas, hippos, and elephants, among others, inhabiting the continent alongside the ancestors of more familiar European animals.

Such animals prospered during the warmer gaps between the Ice Ages, and this, the last full interglacial, was no exception. The phrase 'hippos in the Thames' is often used when talking about this time, and its perfectly accurate. The climate of the day was, if anything, slightly warmer than it is now, with the ice retreating far into the Arctic. All that melting ice had to go somewhere, of course, and the sites of modern day coastal cities such as Amsterdam and Copenhagen would have been underwater. On land, much of the northern continent was covered by dense oak forests, a green wilderness yet to be cleared to make way for farmland or towns.
This interglacial period was, in many ways, not that different to that which had preceded it, with much the same wildlife. Indeed, much of our knowledge of interglacials comes from studying it, for the simple reason that it was the most recent, and the one with the best preserved remains. However, it did not last long - perhaps just 15,000 years. That's a long time from a human perspective; on our timescale of one minute per decade, it's a little over a day. But, when you consider how many months the Pleistocene as a whole lasts on that scale, one day isn't really very much.

It was followed, of course, by the last Ice Age. Again, this is the one we know most about, and, in fact, we have a very detailed understanding of how temperature changed during its course - more so than any of its predecessors. Within the Ice Age, we can see two periods of particularly intense and prolonged cold, lasting thousands of years each. Between these periods, and either side of them, temperatures were more variable, and there were even brief gaps when the ice retreated for maybe a thousand years or so before coming back. Of course, that's a relative thing, and the ice never retreated very far - even at its best, Europe was considerably colder than it is now, and much of the north was trapped under ice sheets for the entire duration.

By many estimates, this was the most severe of all the Pleistocene Ice Ages, and the one that has done the most to shape our perceptions of what they were really like. It was also the blast of cold that finally wiped out the 'African' wildlife that, until then, had been such a feature of European landscapes.

Northern Europe, and mountain ranges, such as the Alps and Pyrenees, were covered in vast glaciers. They would have looked much like modern day Greenland, with some wildlife near the coast, but essentially nothing in the harsh interior where there was no food to eat. Further south, however, it was a different matter. We always think of the Ice Ages as being a time when snow was everywhere, but, for much of Europe, that would only have been true in the winter. In the summer, grass and flowers would have carpeted the ground, and animals would have come out of hibernation. However, this was a tundra environment, with permanently frozen subsoil preventing any trees from growing, presenting a bleak and open landscape for much of the year. Only along the coasts of Italy, Spain, Portugal, and perhaps Greece, would there have been even sparse pine forests, and they were likely quite marginal.

But, despite the cold, Europe beyond the ice sheets proper was hardly devoid of wildlife. Marmots and lemmings prospered, as did a number of other animals, many of them indistinguishable from those that live in the far north today. Reindeer seem to have been plentiful across much of Europe, for example, alongside musk oxen, ibex, and moose. But, of course, it is the really large animals that capture our imagination, and, of these, none is more iconic than the woolly mammoth.

The hippos and forest-dwelling elephants of the interglacials had always managed to survive through previous Ice Ages by heading south and returning when the warm weather came back. They made it for a surprising way through this one, too, struggling on for a third of the way through the last Ice Age. They only finally died out around 70,000 years ago, when the first of the two exceptionally cold phases began. In earlier times, they had been living alongside the steppe mammoth, an exceptionally large species that was already adapted to colder, more northerly climes.

Towards the end of the second-to-last Ice Age, the steppe mammoths had been replaced by the first woolly mammoths (Mammuthus primigenius). Woolly mammoths originated in Siberia, perhaps as far back as 0.6 million years ago. They seem to have remained there during the interglacials, while the weather in Europe was too warm for them to be comfortable, and to have returned to that continent during the height of the last Ice Age.

Woolly mammoths were smaller than the steppe mammoths had been, although still comparable with modern African elephants. They were, however, better adapted to the cold than their predecessors (and presumed ancestors). Most obviously, of course, they had a thick layer of fur covering most of their bodies, but they also had a layer of blubber under the skin that would have helped keep them warm, and a broad 'hump' of fat on the shoulders. We know this because we don't just have fossils of woolly mammoths to go on, but actual preserved bodies as well.

The fur of frozen mammoth carcases is orange-red in colour, and older pictures of the animals used this as a guide when colouring them in. However, we've known for a while that this probably isn't the colour they actually were in life, and that it's more a result of slow chemical degradation during their time entombed in ice. In reality, they were probably dark brown, or even black. A genetic study from 2006 also shows that a minority have been a much paler, straw colour, although more recent work implies that such individuals would have been so rare as to be almost non-existent.

We know from the fossils that woolly mammoths had teeth perfectly
adapted to grinding up the sort of tough vegetation that is all that can
survive in harsh tundra environments. While steppe mammoths had similar
features, they were not so extreme, which may explain why they were replaced. But,
again, we don't need to rely just on fossils to tell us this, because
some of those preserved bodies still contained the remnants of the
animal's last meal. From these, we know that they fed primarily on grass and sedges - which is, after all, what there's most of in the tundra - but also on flowers and shrubs such as dwarf birch
when they were available. The ivory of their tusks also shows scratch
marks that suggest mammoths may have used them to scrape up soil to get at roots
and tubers.

Nor are we just restricted to their diet and preferred habitat when it comes to reconstructing the lives of woolly mammoths. Excavations in Siberia in the late 1980s uncovered what appears to be an entire family group of the animals that died together. The group includes a number of adults, juveniles, and calves as young as one month (plus a foetus, meaning that one of the females was pregnant). Taken together, these imply that woolly mammoths lived in herds of similar size and composition to those of modern African elephants.

The preserved bodies also enable us to perform genetic analyses on woolly mammoths, similar to those we do regularly on living species. Although, as of this writing, the full genome of the woolly mammoth has yet to be sequenced, we do have some significant proportions, including the entire mitochondrial genome, something widely used to determine evolutionary relationships between species. This reveals that woolly mammoths were probably more closely related to living Asian elephants than to African ones. It's hard to know for sure, though, partly because the evidence is still incomplete, but also because the three lineages diverged from one another over what seems to have been a relatively short period of time - making the exact sequence of events difficult to resolve.

Using the same evidence, our best guess for the origins of mammoths as a group distinct from other elephants places it around 4 million years ago. That's well before the Pleistocene, but remember that we're talking about all mammoths here, not just the woolly sort, which, as mentioned above, are, at best, only 0.6 million years old. There's also some evidence from the genetics that there may have been two notably different populations of woolly mammoth living alongside one another - these might, perhaps, have been different subspecies.

So well adapted were woolly mammoths to their frozen environment that they only rarely reached the southernmost parts of Europe, where the tundra began to give way to woodland. When the world eventually began to warm again, as the last Ice Age approached its end, the woolly mammoths once again retreated to Siberia. They returned to Europe briefly during the Younger Dryas, the very last gasp of the Ice Age, before retreating once again, never to come back.

Even so, while many of their contemporaries died out at the end of the Pleistocene, woolly mammoths did not, surviving into the current, Holocene, epoch. The last two thousand years of the Pleistocene had seen their range contract, however: they vanished from southern Siberia around 10,500 years ago, and from the northernmost parts of the mainland around a thousand years after that, at the official end-point of the Pleistocene. By the Holocene, the only woolly mammoths left were on islands in the Arctic Ocean. It had been thought that, in an example of insular dwarfism, these last mammoths became smaller in order to survive on the limited food available, but the evidence for this is no longer thought to be very strong.

We know that humans hunted mammoths in North America, but there's far less evidence that they did so in Europe or Asia, which makes it unlikely that were a major cause in the extinction of the species. Instead, it seems more plausible that it was simply climate change that ended their existence. Without vast plains of Alpine tundra on which to roam, woolly mammoths simply couldn't survive.

The very last woolly mammoth died around 1700 BC, on Wrangel Island. That's a remarkably late date for something associated with the Ice Ages, and makes the last members of the species much younger than the pyramids.